Re: [PATCH] mmc: dts: am654: Add 650ms power off delay for proper MMC power cycle
From: Judith Mendez
Date: Tue Sep 08 2026 - 14:36:45 EST
Hi Nishanth,
On 9/8/26 12:49, Nishanth Menon wrote:
On 12:29-20260908, Judith Mendez wrote:
Hi Nishanth,
On 9/8/26 12:16, Nishanth Menon wrote:
On 12:06-20260908, Judith Mendez wrote:
The AM65x EVM MMC1 VDD rail discharges slowly through RC circuit.
SD spec (section 6.4.1.3) requires VDD < 0.5V for minimum 1ms during
power cycle. Measured discharge time on this board is ~632.97ms to
reach < 0.5V threshold.
During boot, MMC power cycles between boot stages. Without power off
delay, VDD doesn't fully discharge. Card retains 1.8V signaling state
from previous stages. When U-Boot initializes, card no longer asserts
S18A in ACMD41 response (already switched to 1.8V). Host misinterprets
card state and forces 1.8V anyway, falling into a bad state and causing
a boot failure.
Add power-off-delay-us = 650000 (650ms) to ensure proper VDD discharge
during power cycles, allowing card to fully reset state between boot
stages.
Signed-off-by: Judith Mendez <jm@xxxxxx>
Fixes ?
Fix is really for u-boot, don't think I can point to a u-boot
commit here can I?
You'd face the same problem if you are starting from a different boot
media and then enable MMC in linux kernel, right? what was described in
the patch sounds like a discharge latency of the evm. This was missed in
the original commit?
Not exactly. The failure is only seen at u-boot specifically with
SD card boot. Very specifically due to the MMC driver in u-boot and
commit [0] which is not reverted since it is technically correct.
The commit just happened to expose our EVM's issue first. According to
the discussion, mirror Linux patch may be coming soon, which we are
now prepared for with this patch and [1].
[0] https://lore.kernel.org/u-boot/d17a810c-e2db-4a24-a37d-2d7e02466ce9@xxxxxx/
[1] https://lore.kernel.org/linux-mmc/20260806162202.657512-1-jm@xxxxxx/
~ Judith